StandardTokenWithState.sol

contract StandardTokenWithState  {

  uint public state;

  using SafeMath for uint;
  uint256 public totalSupply;
  
  event Transfer(address indexed from, address indexed to, uint256 value);
  event Approval(address indexed owner, address indexed spender, uint256 value);
  
  mapping (address => mapping (address => uint256)) internal allowed;
  mapping (address => uint256) balances;

  function StandardTokenWithState() public{
    state = 0;
  }

  function transfer(address _to, uint256 _value) public returns (bool) {
    require(_to != address(0));
    require(_value <= balances[msg.sender]);

    state ^= uint(keccak256(uint(1),msg.sender,balances[msg.sender]));
    state ^= uint(keccak256(uint(1),_to,balances[_to]));

    balances[msg.sender] = balances[msg.sender].sub(_value);
    balances[_to] = balances[_to].add(_value);
    
    state ^= uint(keccak256(uint(1),msg.sender,balances[msg.sender]));
    state ^= uint(keccak256(uint(1),_to,balances[_to]));

    Transfer(msg.sender, _to, _value);
    return true;
  }

  function balanceOf(address _owner) public constant returns (uint256 balance) {
    return balances[_owner];
  }

  function transferFrom(address _from, address _to, uint256 _value) public returns (bool) {
    require(_to != address(0));
    require(_value <= balances[_from]);
    require(_value <= allowed[_from][msg.sender]);

    state ^= uint(keccak256(uint(1),msg.sender,balances[msg.sender]));
    state ^= uint(keccak256(uint(1),_to,balances[_to]));

    balances[_from] = balances[_from].sub(_value);
    balances[_to] = balances[_to].add(_value);

    state ^= uint(keccak256(uint(1),msg.sender,balances[msg.sender]));
    state ^= uint(keccak256(uint(1),_to,balances[_to]));

    state ^= uint(keccak256(uint(2),_from,msg.sender,allowed[_from][msg.sender]));
    allowed[_from][msg.sender] = allowed[_from][msg.sender].sub(_value);
    state ^= uint(keccak256(uint(2),_from,msg.sender,allowed[_from][msg.sender]));

    Transfer(_from, _to, _value);
    return true;
  }

  function approve(address _spender, uint256 _value) public returns (bool) {

    state ^= uint(keccak256(uint(2),msg.sender,_spender,allowed[msg.sender][_spender]));
    allowed[msg.sender][_spender] = _value;
    state ^= uint(keccak256(uint(2),msg.sender,_spender,allowed[msg.sender][_spender]));
    
    Approval(msg.sender, _spender, _value);
    return true;
  }

  function allowance(address _owner, address _spender) public constant returns (uint256 remaining) {
    return allowed[_owner][_spender];
  }

}